As a relatively straight-to-the-point kind of guy, Sandburg had his share of critics that expressed some difficulties getting around the fact that he just doesn't feel the need to create neatly packaged forms and meters for his work. Some thought it was too simple, while others praised him for his simplicity. Sandburg simply thought that if a rhyme works, then sure: use it; if not, no big deal.
So it's no surprise that "Fog" is free in terms of meter and form. In fact, all of the enjambment we see maintains the poem's flow between the movement of its extended metaphor and sets of images that likewise have some movement to them. Take a look again at lines 1 and 2 that blend everything together into one solid image of that creeping cat-fog. It's almost as if we're moving with the fog and feel the passing of time between its approach ("comes on little cat feet") and its arrival ("it sits looking").
The movement between line breaks also contributes to the poem's transition between moods, beginning with an anticipation that leads us to a sort of anxious uncertainty, and then ends with relief as the fog "moves on."
All in all, no matter how simple and short Sandburg's poem may be, he nevertheless manages to say a lot with only a few words without resorting to conventional techniques of form and meter. It's as if the poem's simplicity is also seen in its form without any need for fancy devices. Pretty impressive, no?